Abstract
Highly twisted magnetic flux ropes, with finite length, are subject to kink instabilities, and could lead to a number of eruptive phenomena in the solar atmosphere, including flares, coronal mass ejections (CMEs) and coronal jets. The kink instability threshold, which is the maximum twist a kink-stable magnetic flux rope could contain, has been widely studied in analytical models and numerical simulations, but still needs to be examined by observations. In this article, we will study twists released by 30 off-limb rotational solar coronal jets, and compare the observational findings with theoretical kink instability thresholds. We have found that: 1) the number of events with more twist release becomes less; 2) each of the studied jets has released a twist number of at least 1.3 turns (a twist angle of 2.6$\pi$); and 3) the size of a jet is highly related to its twist pitch instead of twist number. Our results suggest that the kink instability threshold in the solar atmosphere should not be a constant. The found lower limit of twist number of 1.3 turns should be merely a necessary but not a sufficient condition for a finite solar magnetic flux rope to become kink unstable.
Highlights
Eruption of solar magnetic flux ropes has been considered as one of the main drivers of the so-called “space weather”
In the simulations, the pre-eruption twisted magnetic flux rope is not directly involved in the eruption of the non-rotational jet. We have found it difficult to directly compare the twist released by a rotational jet and stored in its pre-reconnection flux rope from observations, because: (1) for an off-limb jet, we do not have accurate vector magnetic field observations to investigate the underlying magnetic flux rope; and (2) for an on-disk jet, even though we can study the twist stored in the underlying magnetic flux rope using magnetic field extrapolations, it is hard to investigate the rotational motion of the jet using imaging observations and spectral observations with sufficiently enough spectral resolution at the needed temperatures
In this research, using high spatial- and temporal-resolution observations obtained at the SDO/AIA 304 Å passband, we have studied the detailed temporal and spatial evolution, especially the rotational motion, of 30 off-limb rotational solar coronal jets that had erupted between 2010 and 2017
Summary
Eruption of solar magnetic flux ropes (see reviews in e.g., Raouafi, 2009; Schrijver, 2009; Chen, 2011; Filippov et al, 2015a; Karpen, 2015) has been considered as one of the main drivers of the so-called “space weather”. The physical scenario of “untwisting” jets is usually described as follows: a newly emerging (e.g., observations in Liu et al, 2016b; Zheng et al, 2018) or a pre-existing closed flux system (disturbed by footpoint motions, e.g., observations in Chen et al, 2017) reconnects with the ambient open magnetic field, during which twists contained in the closed flux system could be passed into the open fields and are released during the rotational motion of the associated jet Following this idea, a natural question may be raised: will all the twists stored in the pre-reconnection flux rope be released during the coronal jet eruption? Though the possible scenario of partial eruption was not included in their simulations, we have found, through a detailed observational and numerical study of solar coronal twin jets (Liu et al, 2016a), that the residual twist remaining after the jet eruption is very small when compared to the total twist stored in the pre-eruption magnetic flux rope.
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